There are lots of different ways- here are some of the simplest:
-Give your characters different class backgrounds, so that they can use different levels of formality (different titles, contractions/lack of contractions, etc.) -Have them notice things differently. Maybe one character automatically identifies half the trees she passes by species, while to another they're just "trees." On the other hand, that second character may catch expressions and nuances on people's faces that the first can't. -Give them different basic attitudes towards the world. There's a tendency for a lot of first-person narrators to tend towards cynicism and sarcasm. Combat that with one who's essentially solemn, or self-satisfied, or joyful. -Use different catchphrases or characteristic expressions. (Just be careful not to overdo it; a little of this goes a long way). -Make them comfortable around different kinds of people. Maybe one relaxes around soldiers, another around women; this means that even if they're close together in the midst of an army or the midst of a harem, their reactions should still be distinct.